1. Field of the Invention
This invention relates to the preparation of stable concentrated solutions of oxime metal extractants such as aldoximes and ketoximes for supplying metal extraction system operators. More specifically it relates to stable concentrates of hydroxy aryl oxime metal extractant reagent compositions, either individual ketoximes or aldoximes or mixtures thereof in varying ratios by weight of 1:100 to 100:1 aldoximes to ketoxime. The concentrates are solutions of the individual hydroxy aryl oxime either individually ketoxime or aldoxime, or mixtures thereof in a water immiscible hydrocarbon solvent, such as kerosene, or modifiers, or mixtures thereof. The concentrates are flowable, pourable, pumpable and maintain their stability in shipping to the extraction plant sites and in storage.
As is well known to those skilled in the art in relation to extraction systems operations, as exemplified in U.S. Pat. No. 4,582,689 and 4,507,268, the starting material for large scale solvent extraction processing of copper is an aqueous solution generally obtained by leaching a body of ore, which contains a mixture of metals including copper. The leaching medium dissolves salts of copper and other metals as it trickles through the ore to provide an aqueous solution of the mixture of metal values. The metal values are usually leached with a sulfuric acid medium providing an aqueous acidic solution, but the ore may sometimes be leached with an aqueous ammoniacal solution to provide a leach liquor of a basic aqueous solution.
In the extraction system operation the aqueous solution is mixed in a large mixer tank with an extraction reagent which is dissolved in a water immiscible organic hydrocarbon solvent, such as kerosene to give a dilute solution containing about 5% by weight to about 40% by weight of the extractant reagent to give an organic phase suitable for use in a solvent extraction process. The reagent includes an extractant chemical insoluble in water and soluble in the organic solvent, which selectively forms a metal-extractant complex with the copper ions in preference to other ions of other metals. The step of forming the complex is called the extraction or loading stage of the solvent extraction process.
The outlet of the mixer continuously feeds in a continuous process to a large settling tank. While reference is made to a continuous process, the operation may be carried out in a batch basis, if desired. In the large settling tank, the organic phase now containing the copper-extractant complex in solution is separated from the copper-depleted aqueous solution (raffinate phase). This part of the process is called the phase separation. Usually the process of extraction is repeated through two or more mixer settler stages in order to more completely extract the desired metal.
Where two or more mixer-settler stages are employed for extraction, countercurrent flow of the feedstock aqueous solution and the organic phase or reagent solution is employed. In a typical 3-stage extraction system, for example, the feedstock will flow through an initial mixer-settler (E-1), subsequently though a second stage (E-2) and then through a final mixer-settler stage (E-3). As a result, by the time the feedstock reaches mixer-settler stage E-3, much of the extractant will be in the form of a copper extractant complex and the organic phase will be contacting the feedstock solution when it is in condition wherein little, if any of the dissolved copper remains therein.
After extraction, the depleted aqueous feedstock (raffinate) is either discharged or recirculated to the ore body for further leaching. The loaded organic phase containing the dissolved copper-extractant complex is fed to another set of mixer-settler tanks, where it is mixed with an aqueous strip solution, such as highly acidic sulfuric acid solution. The highly acidic strip solution breaks apart the copper-extractant complex and permits the purified and concentrated copper to pass to the aqueous strip solution. As in the extraction process described above, the mixture is fed to another mixer-settler tank for phase separation. This process of breaking the copper-extractant complex is called the stripping stage, and the stripping operation is repeated through two or more mixer-settler stages to more completely strip the copper from the organic phase.
From the stripping settler tank, the regenerated stripped organic phase is recycled to the extraction mixers to begin extraction again, and the strip aqueous phase is customarily fed to an electrowinning tankhouse, where the copper metal values are deposited on plates by a process of electrodeposition. After electrowinning of the copper values from the aqueous strip solution, the solution known as spent electrolyte is returned to the stripping mixers to begin stripping again.
As is apparent, the extraction process operations requires large volumes of extraction reagent and aqueous leach and stripping solutions. In commercial operations, the mixer/settler tanks are large tanks. One such mixer tank in a commercial operation employs a tank on the order of about 28 meters by 28 meters (or about 92 feet long and 92 feet wide). With an organic phase extractant level of about one foot deep, the total volume of organic phase contained in the mixer/settler tank thereof would be about 8464 cubic feet, which converted to liters would be a volume greater than 200,000 liters. Modern solvent extraction plants typically consist of two stages of extraction, 1 stage of stripping, and an organic phase surge tank which will contain as a minimum at least the volume of organic contained in one mixer/settler tank. This corresponds to a total organic phase volume of 800,000 liters. At typical extraction reagent concentrations of 15% by volume in the organic phase, one would have to handle 600 drums of reagent as previously supplied, to fill the system. In the prior supply of phenolic oximes to extraction systems operations in a copper recovery process by solvent (SX) processes, from leaching solutions at copper mines or from waste metal treatment solutions, the phenolic oxime extractants in hydrocarbon solvent solutions were supplied to the operations in conventional 200 liter drum containers at approximately 1.5 to 1.8 molar (an oxime content by weight of about 48–61% depending on the particular oxime and its molecular weight). For a ketoxime, such as 2-hydroxy-5-nonyl acetophenoxime, the oxime content by weight is about 48%. For the 5-nonyl salicylaldoxime the oxime content is about 51% by weight and for the 5-dodecyl salicylaldoxime the oxime content is about 61%.
As is apparent, in view of the large volume employed in the extraction systems operations, this required a large number of 200 liter drum containers with attendant problems of logistics, handling and worker exposure.
It is also known that these same oxime compounds, when present at or near 100% solids, present both logistic and health hazard problems as follows:
(a) first, at 100% solids these compounds are extremely viscous, such that they will not pour or are not pumpable at ambient temperatures. Heat is then required to remove the material from its container, whether that container be a small bottle, a drum, or a large storage tank. Dilution with a hydrocarbon solvent will diminish the viscosity problem.
(b) Second, when heat is applied to these compounds, it can trigger an exothermic autocatalytic decomposition process which can result in pressure buildup in a sealed vessel and potential rupture of the vessel. Surprisingly, the sensitivity of these materials to heat is highly dependent upon their concentration in the hydrocarbon solvent/modifier and the total volume of mass in the storage container.
(c) Third, small 200 liter drum containers do not represent as great a problem with regard to the decomposition. Because of their small volume, the heat generated in the center of a small container, such as a drum, has a short distance to travel through the surrounding liquid to reach a surface, where radiation can occur so as to cool the contents of the drum preventing heating of the drum's contents to the point where a run-away degradation occurs. However in large volume containers, such as the 1 cubic meter (1000 liters) volume in a liquid insulated bulk container (LIBC) or an “isotainer”(20,000 liters), there is reduced overall surface area to volume ratio of the container, so that heat generated by the concentrate therein is not so readily dissipated as in the smaller 200 liter drum container and the temperature of the material can easily reach the point that the rate at which heat is generated by the decomposition reaction exceeds the ability of the system to cool itself by radiating heat to the environment resulting in a rapid increase in the rate of the decomposition reaction and resulting in a run-away reaction. This temperature is dependent on the concentration of the material and the size of the storage vessel. It is referred to as the temperature of no return.